1. Field of the Invention
The present invention is directed to a method for applying a stray radiation grid having detector elements arranged in a matrix, and is also directed to an X-ray detector having a stray radiation grid that applied by such a method.
2. Description of the Prior Art
In X-ray image technology, high demands are currently made of the image quality of the X-ray exposures. For making such exposures, particularly as implemented in medical X-ray diagnostics, a subject to be examined is transirradiated by X-rays of an approximately punctiform X-ray source, and the attenuation distribution of the X-rays is two-dimensionally acquired at that side of the subject opposite the X-ray source. A line-by-line acquisition of the X-rays attenuated by the subject also can be undertaken, for example in computed tomography installations. In addition to X-ray films and gas detectors, solid-state detectors are being increasingly utilized, these usually having a matrix-like arrangement of optoelectronic semiconductor components as optoelectrical receivers. Ideally, each picture element of the X-ray exposure should correspond to the attenuation of the X-rays by the subject on a straight-line axis from the punctiform X-ray source to a location at the detector surface corresponding to the picture element. X-rays that are incident on the X-ray detector that proceed on such a straight-line axis from the punctiform X-ray source are referred to as primary rays.
Due to unavoidable interactions, however, the X-rays emanating from the X-ray source are scattered in the subject, so that scattered rays, referred to as secondary rays, are also incident onto the detector in addition to the primary rays. These scattered rays, which can cause up to more than 90% of the overall signal modulation of an X-ray detector in diagnostic images dependent on properties of the subject, represent an additional noise source and therefore diminish the recognizability of fine contrast differences. This serious disadvantage of the stray radiation is due to a significant, additional noise component in the image exposure caused by the quantum property of the stray radiation.
For reducing the stray radiation incident on the detector, a stray radiation grid is introduced between the subject and the detector. Stray radiation grids are composed of regularly arranged structures that absorb X-rays and between which through channels or through slots are fashioned for the optimally unattenuated passage of the primary radiation. Given focused stray radiation grids, these through channels or through slots are directed toward the focus in conformity with the distance from the punctiform X-ray source, i.e. the distance from the focus of the X-ray tube. In unfocussed stray radiation grids, the through channels or through slots are arranged over the entire surface of the stray radiation grid perpendicular to the surface thereof. This, however, leads to a noticeable loss of primary radiation at the edges of the image exposure since a larger part of the incident primary radiation strikes the absorbent regions of the stray radiation grid at these locations.
Extremely high demands are made on the properties of X-ray stray radiation grids for achieving a high image quality. The scattered rays should be absorbed as well as possible; however, as much of the primary radiation as possible should pass through the stray radiation grid unattenuated. A reduction of the scattered rays incident on the detector surface can be achieved by a large ratio of the height of the stray radiation grid to the thickness or the diameter of the through channels or through slots, i.e. by a high shaft ratio. However, image disturbances due to absorption of a part of the primary radiation can occur because of the thickness of the absorbent structure or wall elements lying between the through channels or through slots. Especially given employment of solid-state detectors, inhomogeneities of the grid, i.e. deviations of the absorbent regions from their ideal position, lead to image disturbance due to an imaging of the grid in the X-ray image.
For minimizing image disturbances due to stray radiation grids, it is known to move the grids in the lateral direction during the exposure. Given extremely short exposure times of, for example, 1-3 ms, however, stripes still can occur in the image due to an inadequate motion velocity of the grids. Disturbing stripes due to the reversal of the motion direction of the grids during the exposure also can occur given very long exposure times.
Solid-state detectors that are formed of a number of detector elements arranged in a matrix are being recently increasingly utilized for X-ray image exposures. The detector elements are arranged in a grid that is usually quadratic or rectangular. The incidence of scattered rays onto the detector surface formed by the detector elements also must be reduced as far as possible in such solid-state detector by means of effective suppression measures. Due to the regular structure of the picture elements of the detector formed by the detector elements, there is the additional risk that the structures of picture elements and stray radiation grids interfere with one another. Disturbing Moiré phenomena can occur as a result. These can be minimized or eliminated by a subsequent image processing measure in certain instances. This, however, is possible only when their projection image on the detector is absolutely invariable.
U.S. Pat No. 6,021,173 discloses an approach that is intended to avoid Moiré structures during operation of an X-ray detector with detector elements arranged in a matrix in conjunction with a stationarily arranged stray radiation grid. In this patent, the stray radiation grid is applied over the detector surface directly on the X-ray detector. The absorbent structural elements of the stray radiation grid are disposed at a distance from one another that is less than the expanse of the smallest resolvable detail in the X-ray image. The regularly arranged, absorbent structure elements therefore are imaged with such a high spatial frequency that they lie beyond the resolution of the X-ray detector. Since the spacing of the structural elements in the stray radiation grid cannot be selected arbitrarily small, a detector having an adapted, limited spatial resolution must be employed. This, however, leads to an undesirable reduction of the detective quantum efficiency (DQE) given high spatial frequencies.